As an example of an optical communication system, there is a passive optical network (PON) system. The PON system is provided with an optical multiplexer and demultiplexer (MUX/DMUX) that is an optical passive component. The optical MUX/DMUX is arranged between an optical line terminal (OLT) provided in a station such as a service provider and an optical network unit (ONU) provided in a house of a subscriber.
The optical MUX/DMUX is connected to the OLT with an optical fiber transmission line, splits a signal transmitted from the OLT into a plurality of signals according to the number of ONUs, and transmits the split signals to the respective ONUs. On the other hand, the optical signal transmitted from the respective ONUs are combined by the optical MUX/DMUX and transmitted to the OLT.
In recent years, as communication traffic is steeply increased and the number of subscribers is increased, the PON system is requested to increase in the number of optical splitting by the optical MUX/DMUX and to expand (or increase) a transmission distance and speed of an optical signal. In order to satisfy the request, it may be effective to apply an optical amplifier to the PON system. As an example of a technology relating to the PON system and the optical amplifier, there are technologies disclosed in JP 2004-288702 A, JP 2010-252334 A, JP 2011-109248 A, and JP 2012-222170 A.
In the PON system, an optical amplifier is applicable to one or both of the OLT and the ONU corresponding to terminal station(s) or a section between the OLT and the ONU.
However, in a case where an optical amplifier is applied to the OLT (or the ONU) corresponding to an optical transmission terminal station in order to preliminarily compensate transmission line loss and optical split loss at the transmission station, a significantly large optical power may be necessarily input to an optical fiber transmission line according to the optical transmission distance and the number of subscribers. As a result, the waveform of the optical signal may be deteriorated due to a non-linear effect of the optical fiber transmission line and a reception performance of the optical signal may be decreased.
Further, since the optical amplifier has an upper limit of the optical output power, a shortage in the output optical power may be possibly occurred depending on the optical transmission distance and the number of subscribers. Furthermore, in a case where an optical amplifier is applied to the ONU to amplify a signal light to be transmitted to the OLT, the optical amplifier would be necessarily provided for every ONU (that is, the number of subscribers).
Meanwhile, in a case where an optical amplifier is applied to the ONU (or the OLT) corresponding to an optical reception terminal station in order to compensate the transmission line loss and the optical split loss at the reception station, since a reception light being passively-damped in optical power is input to the reception optical amplifier, noise light component occurred in the reception optical amplifier would be increased, and therefore, the reception performance would be degraded. Further, in a case where an optical amplifier to amplify a reception light from the OLT is applied to the ONU, the optical amplifier would be necessarily provided for every ONU similarly to the application to the optical transmission terminal station. In order to reduce the degradation in the reception performance due to the noise light component, an optical filter to cut off the noise light component is also applicable together with the optical amplifier. However, in the case where the optical amplifier is provided in the ONU, a set of the optical amplifier and the optical filter would be necessarily provided for every ONU.
As described above, upon considering that the optical amplifier is applied to the PON system, an approach to apply an optical amplifier to the optical transmission terminal station or the optical reception terminal station has room for improvement in the reception performance and the cost. An increase in cost is not preferable for the PON system because it has one of advantageous sales points in the configuration using optical passive components as many as possible.
Therefore, in order to increase the number of optical splitting or to extend the transmission distance in the PON system, it is considered to be useful or effective for applying an optical amplifier to a section between the terminal stations, for example, to the optical fiber transmission line between the OLT and the optical splitter.
In this case, since the transmission lights transmitted by a plurality of ONUs can be collectively amplified by a single optical amplifier, it is not necessary to provide the optical amplifier for every ONU (or subscriber). Further, even when the number of subscribers is further increased, no additional optical amplifiers is required. Examples of the applicable optical amplifier may be a rare-earth-doped optical fiber amplifier such as an erbium-doped optical fiber amplifier (EDFA), a Raman amplifier, and a semiconductor optical amplifier (SOA), or the like.
However, in a case where an optical amplifier is applied to the section between the terminal stations, an arrangement of a pump light source and a power supply system is one of considerations. For example, since the SOA is operable to perform an amplification by receiving a power supply, the power supply system is required. Further, since each of the Raman amplifier and the rare-earth-doped optical fiber amplifier is operable to perform an optical amplification by receiving a pump light from the pump light source such as a semiconductor laser that is operated by receiving a power supply, the power supply system for the pump light source is required.
Therefore, an approach to provide the pump light source and the power supply system in the section between the terminal stations may impair the advantage of the PON system that can be configured by using passive components only. Further, a large-scaled power supply system depending on the transmission distance and the number of subscribers may be required. Furthermore, a maintenance cost for the power supply system may be increased, and the maintenance of the power supply system may be difficult depending on the installation place of the power supply system. As a result, the reliability of the PON system may be decreased.